In the course of drilling oil or gas wells, whether on land or at subsea locations, blowout preventers (BOPs) are installed on the wellhead shortly after the initial portion of a well has been drilled and initial well casing has been installed in the well below the wellhead. Further well formation operations are performed through the BOPs.
A BOP is a set of opposing hydraulic rams. The rams are located on opposite sides of a central bore through a housing in which the rams and related devices are contained. The fundamental use of a BOP is to suitably close the bore in the BOP housing and thereby seal the well from flow of fluids out of the well in a situation in which the well experiences a high pressure event which cannot be controlled or regulated by other techniques or processes. A high pressure event typically occurs where the well bore is advanced into a geologic zone containing oil or gas at a pressure which is above the bore pressure established by a column of drilling mud presented to the zone. The drilling mud is presented to the zone via a string of drill pipe which extends through the bores of the BOPs into the well to a drill bit at the lower end of the drill pipe string. In such a situation, the zone pressure forces the drilling mud upwardly out of the well. If oil or gas at the zone pressure is allowed to flow past the wellhead, an incident which is hazardous to the environment and to personnel working on the well can occur. When overly high zone pressure is sensed during well drilling, as by a change in the flow of drilling mud at or adjacent the platform where drilling personnel are located, the rams of the BOP are actuated to effect the desired kind of sealing action at the wellhead. Different kinds of sealing action are possible, depending upon the construction details of a BOP.
Well drilling involves the use of a drill bit to engage and penetrate the rock or other geologic formation in which the well bore is being formed. As noted, the drill bit is carried at the lower end of a length (called a string) of drill pipe which extends upwardly in the well, through the wellhead and one or more BOPs at the wellhead, and to the drilling platform. The drilling platform can be located on land over the wellhead and the BOPs, or it can be located substantially above the wellhead in the case of a subsea well. BOPs can be defined to close the annulus in the BOP around a drill string of specified outer diameter, or to close the BOP bore in the absence of a drill string in that bore, or to shear a drill string in the bore and seal the BOP bore. Because a formation overpressure condition can be encountered at any time during well drilling operations, including times when no drill string extends through the wellhead, current well drilling practices rely upon a series of several BOPs at the wellhead; each BOP is configured for well sealing use in a particular condition. The several BOPs are arranged in a stack with their bores aligned, often with other devices present in the stack. The other devices can include a wellhead connector at the bottom of the stack, and an annular ramless BOP, usually at the top of the stack, which operates as a sphincter in the presence or absence of a drill string extending through it.
Oil and gas wells are being drilled into deeper and deeper formations, via both on-land and subsea wells. Subsea wells are being drilled in greater and greater water depths. Modern offshore drilling facilities, such as drillships, are now rated for drilling in water depths as great as 10,000 feet. In those wells, very high zone pressures can be encountered; zone pressures of 20,000 pounds per square inch or higher are known. To be able to operate against and to contain fluids at such pressures, BOPs are becoming larger and stronger. BOP stacks, including related devices, 30 feet or more in height are increasingly common.
As noted above, ram-type BOPs which close around drill pipe are designed and constructed for use with drill pipe of specified diameter. A BOP stack including rams for one size of pipe may be used with pipe of a different size by changing the pipe engaging rams or parts of the rams. Also, the ram operating mechanisms in a BOP are comparatively complex and require inspection and servicing before the BOP is put into service at a wellhead. Such activities, when performed in a large modern BOP stack, may require the presence of personnel at locations well above the bottom of the stack at heights which can be hazardous. The use of safety harnesses by stack service personnel is known, but has been found to restrict movement of personnel in the performance of their tasks. A need exists for a better way to provide safe support for personnel engaged in activities in the upper portions of large BOP stacks.